Within many areas for chip-removing or cutting machining of metal, including milling, it is desirable that the replaceable milling inserts included in the tools should be strong, have a long service life, provide efficient chip removal, as well as—not the least—include as many usable cutting edges as possible for the application in question. Therefore, within the field of milling, round and double-sided milling inserts are suitable not only in so far as a plurality of cutting edges or cutting edge sections can be formed along each one of the peripheries of the two chip faces, but also in so far as corner-free milling inserts are considerably stronger and more durable than such polygonal milling inserts in which each cutting edge ends in a more or less fragile corner.
Round and double-sided milling inserts, e.g. of the type disclosed in US 2011/0103905 A1 and WO 2010/017859 A1, require, in order to guarantee clearance behind the active chip-removing cutting edge, that the appurtenant seat in the basic body is located in a special so-called tipped-in spatial position in which both an axial tipping-in angle and a radial one are negative, contrary to the positive tipping-in angles that are possible when the milling inserts are single sided and formed with a positive cutting geometry. Generally, negative cutting edges are more blunt-cutting than positive, among other things as a consequence of the material to be removed being pushed in front of the milling insert in the direction of rotation, rather than being lifted out by wedge action, as is the case with positive cutting edges. Disadvantages of previously known double-sided milling inserts having a round basic shape are, therefore, among other things that the chip formation may become difficult to master and that the milling operations produce rough and high sounds; this is something that creates a poor working environment in the plant in question. A further shortcoming of previously known milling tools having round, double-sided milling inserts is the fact that ramping operations cannot be successfully carried out because the envelope so surface of the milling insert, as a consequence of the negative tipping-in position of the milling insert, will collide with the generated surface as soon as a considerable axial feed, in addition to the traditional rectilinear one in planes perpendicular to the rotation axis of the tool, is applied to the tool.
Terminology
Before the invention is described in more detail, in order to provide conceptual clarity, certain fundamental concepts vital to the understanding of the invention should be made clear, and which may vary depending on if they only relate to the shape of the basic body of the tool and of the milling insert, respectively, or to the functions thereof in operation. When a feature for instance is described as “nominal”, the same only relates to the milling insert as such, i.e., without coupling to the basic body of the tool, but if the same is denominated “functional”, the same relates to the assembled state of the tool, i.e., with the milling insert mounted in the seat of the basic body. The concept “chip face” relates to any end of the milling insert between which a circumferential envelope surface extends. The individual chip face may either form an upper side or an under side in its mounted state in the seat of the basic body. In each chip face, there are included a plurality of part surfaces closest to each cutting edge. Henceforth, said part surfaces are denominated “chip surfaces”. Furthermore, the concept “slope angle” relates to the angle at which each one of two edge segments included in the individual cutting edge leans in relation to a reference plane that is touched by the highest situated end points between which the cutting edges extend. In the subsequent text, also the concepts “invertible” and “indexable”, respectively, are found. When the milling insert according to the invention is “inverted”, this means that a previously upwardly exposed chip face is turned downward toward a tangential support surface or bottom in the seat with the purpose of exposing the other chip face upward. When the milling insert is “indexed”, this means that the same—after detachment—is rotated a bit on its own centre axis and then again is fixed in the seat. The object of inversion as well as indexing of the milling insert is, in the usual way, to change into an unused cutting edge, when a previously active cutting edge has been worn out, wherein each individual cutting so edge should assume one and the same spatial position in relation to the basic body.